1. Field of the Invention
This invention relates to an electrophotographic device, particularly an electrophotographic device of the reversal developing system by use of an organic photosemiconductor type photosensitive member.
2. Related Background Art
Electrophotographic processes may be classified largely into two systems of positive developing and reversal developing from the viewpoint of the developing system.
In the basic electrophotographic processes of primary charging of photosensitive member, formation of latent image on the photosensitive surface by image exposure, toner development and cleaning, positive developing effects toner development of the portion on the photosensitivve member where no imagewise exposure is effected, or the portion with relatively smaller dose of photoirradiation, in other words the portion with higher absolute value of the photosensitive member surface potential, while the reversal developing is contrariwise the system of toner development of the lower absolute value of the photosensitive surface potential, and therefore development is performed by use of a toner of the same polarity as primary charging.
In the prior art, the system generally employed as the electrophotographic device is positive developing, but in recent years reversal developing systems have been utilized widely in a printer for microfilm, an electrophotographic system printer by use of laser as the light source (laser printer), etc.
As the photosensitive member for electrophotography, selenium, selenium alloys, cadmium sulfide resin dispersion system, amorphous silicon, organic photosemiconductor (OPC) have been employed, and among them OPC is attracting much attention recently and is going to be practically applied widely for its high productivity and cheap production cost.
Since discovery of specific organic compounds which exhibit photoconductivity, a large number of organic photoconductors have been developed. For example, there have been known organic photoconductive polymers such as poly-N-vinylcarbazole, polyvinylanthracene, etc., low molecular weight organic photoconductors such as carbazole, anthracene, pyrazolines, oxadiazoles, hydrazones, polyarylalkanes, etc., organic pigments or dyes such as phthalocyanine pigments, azo pigments, cyanine dyes, polycyclic quinone pigments, perylene type pigments, indigo dyes, thioindigo dyes, or squaric acid methine dyes, etc.
Particularly, organic pigments and dyes having photoconductivity can be synthesized more easily than inorganic materials, and yet expanded in variation to select compounds exhitibing photoconductivity at appropriate wavelength region, and therefore a large number of photoconductive organic pigments and dyes have been proposed.
For example, as disclosed in U.S. Pat. Nos. 4,123,270, 4,247,614, 4,251,613, 4,251,614, 4,256,821, 4,260,672, 4,268,596, 4,278,747, 4,293,628, an electrophotographic photosensitive member by use of a disazo pigment exhibiting photoconductivity as the charge generation material in the photosensitive layer separated in function into the charge generation layer and the charge transport layer has been known, and since the electrophotographic photosensitive member by use of such organic photoconductive member can be produced by coating by selecting appropriately the binder, it has many advantages that productivity is extremely high and cheap photosensitive members can be provided, and yet the photosensitive wavelength region can be freely controlled by selection of the organic pigment.
The laminated type photosensitive member obtained by laminating a charge transport layer and a charge generation layer composed mainly of the charge generation material is more advantageous in sensitivity and elevation of residual potential after durability test than the single layer type photosensitive member.
In the laminated type photosensitive member, the charge generation layer can be formed by coating a charge generation material such as phthalocyanine pigment, dibenzpyrene pigment, azo pigment, etc. and, if desired, a charge transport layer together with a binder (or without binder) on a substrate, and also can be obtained by forming a vapor deposited film by vacuum vapor deposition device, but for easiness of formation, the coating system is principally practiced presently.
However, here when a charge generation layer on which an organic pigment is coated as the charge generation material is used, local charge injection points are formed on the coated surface due to unevenness of particle size and agglomeration mutually between pigment particles during coating, whereby points of particular large dark decay are formed locally, and therefore when such materials are applied as the photosensitive member to form copied images, the above-mentioned local charge injection points will appear as the image defect.
Particularly, when such photosensitive member is used in a copying machine which performs reversal development the local charge injection points of the charge generation layer by use of the pigment as described above become the points with lower absolute values of surface potential as compared with other points, then these points will become the image defect in black spot by toner development.